“Coiled tubing injectors” are machines for running pipe into and out of well bores. Typically, the pipe is continuous, but injectors can also be used to raise and lower jointed pipe. Continuous pipe is generally referred to as coiled tubing since it is coiled onto a large reel when it is not in a well bore. The terms “tubing” and “pipe” are, when not modified by “continuous,” “coiled” or “jointed,” synonymous and encompass both continuous pipe, or coiled tubing, and jointed pipe. “Coiled tubing injector” and, shortened, “injector” refer to machines used for running any of these types of pipes or tubing. The name of the machine derives from the fact that it is typically used for coiled tubing and that, in preexisting well bores, the pipe must be literally forced or “injected” into the well through a sliding seal to overcome the pressure of fluid within the well, until the weight of the pipe in the well exceeds the force produced by the pressure acting against the cross-sectional area of the pipe. However, once the weight of the pipe in the well overcomes the pressure, it must be supported by the injector. The process is reversed as the pipe is removed from the well.
Coiled tubing is faster to run into and out of a well bore than conventional jointed or straight pipe and has traditionally been used primarily for circulating fluids into the well and other work over operations, but can be used for drilling. For drilling, a turbine motor is suspended at the end of the tubing and is driven by mud or drilling fluid pumped down the tubing. Coiled tubing has also been used as permanent tubing in production wells. These new uses of coiled tubing have been made possible by larger diameters and stronger pipe.
Examples of coiled tubing injectors include those shown and described in U.S. Pat. Nos. 5,309,990, 6,059,029, and 6,173,769, all of which are incorporated herein by reference.
A conventional coiled tubing injector has two continuous chains, though more than two can be used. The chains are mounted on sprockets to form elongated loops that counter rotate. A drive system applies torque to the sprockets to cause them to rotate, resulting in rotation of the chains. In most injectors, chains are arranged in opposing pairs, with the pipe being held between the chains. Grippers carried by each chain come together on opposite sides of the tubing and are pressed against the tubing. The injector thereby continuously grips a length of the tubing as it is being moved in and out of the well bore. The “grip zone” or “gripping zone” refers to the zone in which grippers come into contact with a length of tubing passing through the injector.
Several different arrangements can be used to push the grippers against the tubing. One common arrangement uses a skate to apply an even force to the back of the grippers as they pass through the grip zone. In one example, each gripper has a cylindrical roller, or multiple rollers with the same axis of rotation, mounted to its back. The rollers roll along a continuous, planar surface formed by the skate as the grippers pass through the gripping zone. By properly positioning the skate with respect to the tubing, the skate can push the grippers against the tubing with force or pressure that is normal to the tubing. In an alternative arrangement rollers are mounted on the skate, and the back of the grippers have a flat or planar surface that ride along the rollers. The axes of the rollers are co-planar, so that the rollers engage the back of the skates in the same plane, thus effectively presenting a planar rolling surface on which the grippers may roll.
A coiled tubing injector applies a normal force to its grippers. The normal force creates through friction an axial force along the longitudinal axis of the tubing. The amount of traction between the grippers and the tubing is determined, at least in part, by the amount of this force. In order to control the amount of the normal force, skates for opposing chains are typically pulled toward each other by a traction system comprising hydraulic pistons or a similar mechanism, thereby forcing the gripper elements against the tubing. Alternatively, skates are pushed toward each other. The force applied by the traction system to the chains, and thus to the tubing against which the chains are pressed, is adjustable to take into account different operating conditions.
If the force at which a traction system for a coiled tubing injector is set is insufficient for any reason, the injector will lose grip on the tubing. When independently driven chains are used in coiled tubing injectors, there is also a risk that one or more of the chains will begin to slip on the tubing before the other. Once a chain begins to slip on the tubing, the type of friction changes from static to dynamic and the traction of the slipping chain is greatly diminished. When grip is lost, damage to the coiled tubing is possible. Damage is more likely the further the tubing is allowed to slip in the injector chains. When the tubing speed increases, it is more difficult to regain grip and the potential of damage to the tubing, machinery, and the well increases.